1. Field of the Invention
This invention relates to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method which are capable of providing diagnostic information about a microscopic bloodstream circulation on a capillary level and a microscopic structure of vascular flow faster than in a capillary in an angiographic echo method using an ultrasonic contrast agent.
2. Description of the Related Art
In ultrasonic diagnosis, heartbeats or the movement of the baby in the womb can be displayed in real time by a simple operation of just applying an ultrasonic probe to the surface of the body. Since ultrasonic diagnosis requires no X rays, it ensures a high safety and can be used in obstetrical service, home care service at home, or the like. Moreover, since an apparatus used in ultrasonic diagnosis is smaller in size than such a diagnostic apparatus as an X-ray apparatus, an X-ray CT scanner, or an MRI machine, bedside examination can be made easily. Accordingly, ultrasonic diagnosis has many advantages.
With the recent commercialization of an intravenous-dosage ultrasonic contrast agent, an angiographic echo method has been used in examining the heart or liver. The angiographic echo method is to enhance a bloodstream signal by injecting an ultrasonic contrast agent into a vein and then evaluate the hemodynamic stability.
In most contrast agents, microbubbles function as an ultrasonic reflection source. Since bubbles are easily destroyed by the mechanical action of an ultrasonic wave because of their delicate base material, even if the ultrasonic wave is irradiated at the normal diagnosis level, the intensity of the signal from the scanning surface decreases.
Therefore, to observe a dynamic behavior of a return current in real time, it is necessary to reduce the destruction of bubbles due to scanning by, for example, lowering the sound pressure of ultrasonic wave. However, when a low-sound-pressure ultrasonic wave is used, the signal-to-noise (S/N) ratio decreases. Therefore, signal processing to compensate for the decrease is needed.
Making use of the characteristic of bubble destruction, the following method has been devised. First, a low-sound-pressure ultrasonic wave is transmitted and received, thereby visualizing the moving state of bubbles flowing into the scanning surface. Next, the transmission and reception of the low-sound-pressure ultrasonic wave are stopped. A high-sound-pressure ultrasonic wave transmitted after the stop destroys all of the bubbles existing at the scanning surface (precisely, in the irradiated volume). Then, the transmission of the high-sound-pressure ultrasonic wave is stopped. A low-sound-pressure ultrasonic wave retransmitted and re-received visualizes bubbles flowing into the scanning surface. This method is known as a replenishment method (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 11-155858).
Ultrasonic images are treated as two-dimensional images. Actually, however, ultrasonic waves are transmitted in a plurality of directions. On the basis of a plurality of pieces of one-dimensional information acquired from the respective directions, a two-dimensional image is created.
In recent years, a method has been developed which limits the transmission area of high-sound-pressure ultrasonic waves to a local range and destroys only the bubbles in the local area of the diagnostic image. In this method, of the blood vessel images drawn in the diagnostic image, only the bubbles flowing through the desired blood vessel selectively disappear. However, even when only the bubbles in the local area of the diagnostic image had been destroyed, an image reflecting the speed and amount of the contrast agent flowing through the individual blood vessels could not be obtained.